ch18.qxd 2/13/2003 3:05 PM Page 145 Metabolism of Glycogen 18 Peter A. Mayes, PhD, DSc, & David A. Bender, PhD BIOMEDICAL IMPORTANCE phatase catalyzes hydrolysis of pyrophosphate to 2 mol of inorganic phosphate, shifting the equilibrium of the Glycogen is the major storage carbohydrate in animals, main reaction by removing one of its products. corresponding to starch in plants; it is a branched poly- α Glycogen synthase catalyzes the formation of a gly- mer of -D-glucose. It occurs mainly in liver (up to 6%) coside bond between C of the activated glucose of and muscle, where it rarely exceeds 1%. However, be- 1 UDPGlc and C4 of a terminal glucose residue of glyco- cause of its greater mass, muscle contains about three to gen, liberating uridine diphosphate (UDP). A preexist- four times as much glycogen as does liver (Table 18–1). ing glycogen molecule, or “glycogen primer,” must be Muscle glycogen is a readily available source of glu- present to initiate this reaction. The glycogen primer cose for glycolysis within the muscle itself. Liver glyco- may in turn be formed on a primer known as glyco- gen functions to store and export glucose to maintain genin, which is a 37-kDa protein that is glycosylated blood glucose between meals. After 12–18 hours of on a specific tyrosine residue by UDPGlc. Further glu- fasting, the liver glycogen is almost totally depleted. cose residues are attached in the 1→4 position to make Glycogen storage diseases are a group of inherited dis- a short chain that is a substrate for glycogen synthase. orders characterized by deficient mobilization of glyco- In skeletal muscle, glycogenin remains attached in the gen or deposition of abnormal forms of glycogen, lead- center of the glycogen molecule (Figure 13–15), ing to muscular weakness or even death. whereas in liver the number of glycogen molecules is greater than the number of glycogenin molecules. GLYCOGENESIS OCCURS MAINLY IN MUSCLE & LIVER Branching Involves Detachment The Pathway of Glycogen Biosynthesis of Existing Glycogen Chains Involves a Special Nucleotide of Glucose The addition of a glucose residue to a preexisting glyco- (Figure 18–1) gen chain, or “primer,” occurs at the nonreducing, outer end of the molecule so that the “branches” of the As in glycolysis, glucose is phosphorylated to glucose glycogen “tree” become elongated as successive 1→4 6-phosphate, catalyzed by hexokinase in muscle and linkages are formed (Figure 18–3). When the chain has glucokinase in liver. Glucose 6-phosphate is isomer- been lengthened to at least 11 glucose residues, branch- ized to glucose 1-phosphate by phosphoglucomutase. ing enzyme transfers a part of the 1→4 chain (at least The enzyme itself is phosphorylated, and the phospho- six glucose residues) to a neighboring chain to form a group takes part in a reversible reaction in which glu- 1→6 linkage, establishing a branch point. The cose 1,6-bisphosphate is an intermediate. Next, glucose branches grow by further additions of 1→4-glucosyl 1-phosphate reacts with uridine triphosphate (UTP) to units and further branching. form the active nucleotide uridine diphosphate glu- cose (UDPGlc)* and pyrophosphate (Figure 18–2), catalyzed by UDPGlc pyrophosphorylase. Pyrophos- GLYCOGENOLYSIS IS NOT THE REVERSE OF GLYCOGENESIS BUT IS A SEPARATE * Other nucleoside diphosphate sugar compounds are known, eg, PATHWAY (Figure 18–1) UDPGal. In addition, the same sugar may be linked to different nucleotides. For example, glucose may be linked to uridine (as Glycogen phosphorylase catalyzes the rate-limiting shown above) as well as to guanosine, thymidine, adenosine, or cy- step in glycogenolysis by promoting the phosphorylytic tidine nucleotides. cleavage by inorganic phosphate (phosphorylysis; cf hy- 145 ch18.qxd 2/13/2003 3:05 PM Page 146 146 / CHAPTER 18 Glycogen → → (1 4 and 1 6 glucosyl units)x BRANCHING ENZYME Pi (1→4 Glucosyl units) x Insulin UDP GLYCOGEN cAMP GLYCOGEN SYNTHASE PHOSPHORYLASE Glycogen primer Glucagon Epinephrine GLUCAN * TRANSFERASE Glycogenin DEBRANCHING Uridine ENZYME disphosphate glucose (UDPGlc) To uronic acid Free glucose from pathway debranching UDPGlc PYROPHOSPHORYLASE enzyme INORGANIC PYROPHOSPHATASE PPi 2 Pi Uridine UDP triphosphate (UTP) Glucose 1-phosphate 2+ Mg PHOSPHOGLUCOMUTASE Glucose 6-phosphate To glycolysis and pentose phosphate pathway H O ADP NUCLEOSIDE 2 DIPHOSPHO- GLUCOSE-6- + ATP ADP Mg2 GLUCOKINASE KINASE PHOSPHATASE Pi ATP Glucose Figure 18–1. Pathway of glycogenesis and of glycogenolysis in the liver. Two high-energy phosphates are used in the incorporation of 1 mol of glucose into glycogen. ᭺+ , stimulation; ᭺− , inhibition. Insulin decreases the level of cAMP only after it has been raised by glucagon or epinephrine—ie, it antagonizes their action. Glucagon is active in heart muscle but not in skeletal muscle. At asterisk: Glucan transferase and debranching enzyme ap- pear to be two separate activities of the same enzyme. Table 18–1. Storage of carbohydrate in postabsorptive normal adult humans (70 kg). drolysis) of the 1→4 linkages of glycogen to yield glu- cose 1-phosphate. The terminal glucosyl residues from the outermost chains of the glycogen molecule are re- Liver glycogen 4.0% = 72 g1 2 moved sequentially until approximately four glucose Muscle glycogen 0.7% = 245 g → Extracellular glucose 0.1% = 10 g3 residues remain on either side of a 1 6 branch (Figure 18–4). Another enzyme (␣-[1v4]v␣-[1v4] glucan 327 g transferase) transfers a trisaccharide unit from one → 1Liver weight 1800 g. branch to the other, exposing the 1 6 branch point. 2Muscle mass 35 kg. Hydrolysis of the 1→6 linkages requires the de- 3Total volume 10 L. branching enzyme. Further phosphorylase action can ch18.qxd 2/13/2003 3:05 PM Page 147 METABOLISM OF GLYCOGEN / 147 O dephosphorylation of enzyme protein in response to 6CH2OH hormone action (Chapter 9). HN H O Uracil Cyclic AMP (cAMP) (Figure 18–5) is formed from H H 1 O N ATP by adenylyl cyclase at the inner surface of cell OH H O O membranes and acts as an intracellular second messen- HO O P O P O CH2 ger in response to hormones such as epinephrine, nor- H OH O – O – O epinephrine, and glucagon. cAMP is hydrolyzed by phosphodiesterase, so terminating hormone action. In HH Ribose liver, insulin increases the activity of phosphodiesterase. HH HO OH Phosphorylase Differs Between Glucose Diphosphate Uridine Liver & Muscle Figure 18–2. Uridine diphosphate glucose (UDPGlc). In liver, one of the serine hydroxyl groups of active phosphorylase a is phosphorylated. It is inactivated by hydrolytic removal of the phosphate by protein phos- phatase-1 to form phosphorylase b. Reactivation re- then proceed. The combined action of phosphorylase quires rephosphorylation catalyzed by phosphorylase and these other enzymes leads to the complete break- kinase. down of glycogen. The reaction catalyzed by phospho- Muscle phosphorylase is distinct from that of liver. It glucomutase is reversible, so that glucose 6-phosphate is a dimer, each monomer containing 1 mol of pyridoxal can be formed from glucose 1-phosphate. In liver (and phosphate (vitamin B6). It is present in two forms: phos- kidney), but not in muscle, there is a specific enzyme, phorylase a, which is phosphorylated and active in either glucose-6-phosphatase, that hydrolyzes glucose the presence or absence of 5′-AMP (its allosteric modi- 6-phosphate, yielding glucose that is exported, leading fier); and phosphorylase b, which is dephosphorylated to an increase in the blood glucose concentration. and active only in the presence of 5′-AMP. This occurs during exercise when the level of 5′-AMP rises, providing, CYCLIC AMP INTEGRATES THE by this mechanism, fuel for the muscle. Phosphorylase a is REGULATION OF GLYCOGENOLYSIS the normal physiologically active form of the enzyme. & GLYCOGENESIS cAMP Activates Muscle Phosphorylase The principal enzymes controlling glycogen metabo- lism—glycogen phosphorylase and glycogen synthase— Phosphorylase in muscle is activated in response to epi- are regulated by allosteric mechanisms and covalent nephrine (Figure 18–6) acting via cAMP. Increasing modifications due to reversible phosphorylation and the concentration of cAMP activates cAMP-dependent 1→4- Glucosidic bond Unlabeled glucose residue 1→6- Glucosidic bond 14C-labeled glucose residue 14C-Glucose → added New 1 6- bond GLYCOGEN BRANCHING SYNTHASE ENZYME Figure 18–3. The biosynthesis of glycogen. The mechanism of branching as revealed by adding 14C-labeled glucose to the diet in the living animal and examining the liver glycogen at further intervals. ch18.qxd 2/13/2003 3:05 PM Page 148 148 / CHAPTER 18 types of subunits—α, β, γ, and δ—in a structure repre- αβγδ α β sented as ( )4. The and subunits contain serine residues that are phosphorylated by cAMP-dependent protein kinase. The δ subunit binds four Ca2+ and is identical to the Ca2+-binding protein calmodulin (Chapter 43). The binding of Ca2+ activates the cat- alytic site of the γ subunit while the molecule remains in the dephosphorylated b configuration. However, the phosphorylated a form is only fully activated in the presence of Ca2+. A second molecule of calmodulin, or TpC (the structurally similar Ca2+-binding protein in muscle), can interact with phosphorylase kinase, caus- ing further activation. Thus, activation of muscle con- PHOSPHORYLASE GLUCAN DEBRANCHING traction and glycogenolysis are carried out by the same TRANSFERASE ENZYME Ca2+-binding protein, ensuring their synchronization. Glucose residues joined by 1 → 4- glucosidic bonds Glycogenolysis in Liver Can Glucose residues joined by Be cAMP-Independent → 1 6- glucosidic bonds In addition to the action of glucagon in causing forma- tion of cAMP and activation of phosphorylase in liver, Figure 18–4. Steps in glycogenolysis. ␣ 1-adrenergic receptors mediate stimulation of glyco- genolysis by epinephrine and norepinephrine. This in- volves a cAMP-independent mobilization of Ca2+ protein kinase, which catalyzes the phosphorylation by from mitochondria into the cytosol, followed by the ATP of inactive phosphorylase kinase b to active stimulation of a Ca2+/calmodulin-sensitive phosphory- phosphorylase kinase a, which in turn, by means of a lase kinase.